[Haskell-cafe] A haskell error message

[Daniel Fischer]

Am Dienstag 21 April 2009 20:52:40 schrieb siso dagbovie:
> Hi,
>
> I have been trying to specify the depth-first numbering algorithm as follow:
> >_dfNum             :: Int -> [Tree a] -> [Tree Int]
> >_dfNum i ((Node n ts):us) = (Node i vs):(_dfNum (i+1+(length ts)) us)
> > where vs=_dfNum (i+1) ts
>
> But when I test it, I receive the message:
>
> Non-exhaustive patterns in function _bfNum
>
> Please, could you give me an idea of how I could fix this?

First, you need to provide a case for the empty list

_dfNum _ [] = []

to tell GHC what to do at the end of the list of trees.

Second, if Tree a has other constructors than Node, you must also provide cases to tell 
GHC what to do when a list-element uses those constructors.